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SYSTEMATIC REVIEW
Stiffness as a Risk Factor for Achilles Tendon Injury in RunningAthletes
Anna V. Lorimer1,2 • Patria A. Hume1
� Springer International Publishing Switzerland 2016
Abstract
Background Overuse injuries are multifactorial resulting
from cumulative loading. Therefore, clear differences
between normal and at-risk individuals may not be present
for individual risk factors. Using a holistic measure that
incorporates many of the identified risk factors, focusing on
multiple joint movement patterns may give better insight
into overuse injuries. Lower body stiffness may provide
such a measure.
Objective To identify how risk factors for Achilles tendon
injuries influence measures of lower body stiffness.
Methods SPORTDiscus, Web of Science, CINAHL and
PubMed were searched for Achilles tendon injury risk
factors related to vertical, leg and joint stiffness in running
athletes.
Results Increased braking force and low surface stiffness,
which were clearly associated with increased risk of
Achilles tendon injuries, were also found to be associated
with increased lower body stiffness. High arches and
increased vertical and propulsive forces were protective for
Achilles tendon injuries and were also associated with
increased lower body stiffness. Risk factors for Achilles
tendon injuries that had unclear associations were also
investigated with the evidence trending towards an increase
in leg stiffness and a decrease in ankle stiffness being
detrimental to Achilles tendon health.
Conclusion Few studies have investigated the link between
lower body stiffness and Achilles injury. High stiffness is
potentially associated with risk factors for Achilles tendon
injuries although some of the evidence is controversial.
Prospective injury studies are needed to confirm this rela-
tionship. Large amounts of high-intensity or high-speed
work or running on soft surfaces such as sand may increase
Achilles injury risk. Coaches and clinicians working with
athletes with new or reoccurring injuries should consider
training practices of the athlete and recommend reducing
speed or sand running if loading is deemed to be excessive.
Key Points
High braking forces and running on soft surfaces
such as sand have previously shown a clear increase
in risk for Achilles tendon injuries and were also
associated with higher lower extremity stiffness
measures.
Factors shown to be protective for Achilles injury
were also associated with higher stiffness measures.
There is a potential link between high lower
extremity stiffness measures and risk for Achilles
injuries, which warrants further prospective
investigation.
Electronic supplementary material The online version of thisarticle (doi:10.1007/s40279-016-0526-9) contains supplementarymaterial, which is available to authorized users.
& Anna V. Lorimer
alorimer@aut.ac.nz; avlorimer@gmail.com
1 Sports Performance Research Institute New Zealand
(SPRINZ) at AUT-Millennium, Auckland University of
Technology, Private Bag 92006, Auckland, New Zealand
2 Unitec Institute of Technology, 139 Carrington Road,
Mt Albert, Auckland 1025, New Zealand
123
Sports Med
DOI 10.1007/s40279-016-0526-9
1 Introduction
Chronic Achilles tendon injuries, commonly referred to as
tendinopathy or tendonitis (from here on referred to as
Achilles injures), are a frustrating injury for athletes owing
to their slow recovery time and tendency for reoccurrence.
When classified for ‘severity’ based on the number of days
off training and prevalence, Achilles injuries were the most
severe lower limb injuries, with only upper back injuries
more severe in elite triathletes [1, 2]. In club and devel-
opment triathletes, Achilles injuries ranked as number one
on the severity scale [2].
Three main themes apparent in the literature regarding
the mechanism of Achilles injury include tensile loading
[3], shearing [4] and hyperthermia [5, 6]. For overuse type
tendon injuries, it has been suggested that ‘non-homolo-
gous loading’ leads to localised tendon damage resulting in
an area of weakened tissue [7]. This area of weakened
tissue gradually becomes larger if loading occurs before
healing is complete, resulting in a cycle of progressively
weakened and dysfunctional tissue. Ex vivo cyclic loading
has shown tendon stiffness decreases over time and hys-
teresis becomes greater, which is believed to indicate
accumulated damage [8]. It is unlikely that the natural
motion of running would produce forces that could lead to
macroscopic disruption of the tendon structure. It is con-
ceivable, however, that injurious movement patterns lead
to microscopic but accumulating damage, which ultimately
results in pain and dysfunction for the athlete. Loading
through the tendon during cycling is significantly lower
compared with running [9–11]. It is therefore more likely
for injury to occur during running. The focus for this
review was therefore directed at running.
Research into the nature of overuse Achilles tendon
injuries is extensive, yet uncertainty remains around how to
identify athletes susceptible to Achilles tendon injury. Our
recent review on Achilles tendon injury risk factors asso-
ciated with running identified two variables, high vertical
forces and high arch, which showed strong evidence for
reduced injury risk. High propulsive forces and running on
stiffer surfaces may also be protective [12]. Only one
biomechanical variable, high braking force, showed clear
evidence for increasing Achilles injury risk. The majority
of the biomechanical risk factors examined showed unclear
results, which may be attributed to the multifactorial nature
of Achilles overuse injuries.
Many risk factors are related to how the athletes’ body
interacts with the environment during gait including
ground reaction forces, muscle activity prior to landing and
immediately post-ground contact and joint motion
throughout stance. The largely inconclusive results for
individual risk factor analysis highlight the need for an
alternative method of assessing injury risk [12]. Dynamical
systems theory suggests that a movement’s end result can
be achieved by multiple movement patterns [13–16]. It is
possible that an injury endpoint does not arise from a single
identifiable factor but from multiple factors working
together to cause eventual breakdown of the tissue [15].
Multiple combinations of multiple factors would give the
observed results for single-risk factor analysis, small-effect
sizes with large confidence intervals. Identifying single
measurements that are influenced by many of the known
risk factors, may provide a better measure of injury risk.
While lower extremity stiffness does not reflect the prop-
erties of the tendon, it does reflect how the different tissues
and joints work together during the first half of stance
phase. Lower extremity stiffness may therefore provide a
means of identifying factors causing stress to the tendon
tissue during functional movements without having to use
more complex localized measures such as tendon stiffness.
Total leg stiffness during hopping had a potentially large
negative effect size (-1.73 ± 1.71) when comparing
Achilles injured and uninjured runners [12]. Single-leg
hopping on a sled apparatus resulted in significantly higher
vertical stiffness in a cohort of Achilles injured patients
compared with healthy volunteers [17]. Stiffness is also
modified by many of the Achilles injury risk factors
identified [12]. A link between individual risk factor
measures and stiffness variables, for example higher leg
stiffness associated with higher braking forces and soft
surfaces, would indicate a potential for the use of stiffness
as a more holistic measure of Achilles injury risk.
Stiffness is a pseudo measure relating to how the lower
body interacts with the ground upon landing. The body is
modelled as a point mass balanced on a compressible
spring while the joints are modelled as torsional springs
with each ‘spring’ having a specific stiffness, k (see Fig. 1)
Fig. 1 Biomechanical stiffness models for changes in (a) vertical andleg stiffness [19], and (b) joint stiffness [18] during running. Vertical
stiffness is calculated from maximum vertical force and centre of
mass (COM) displacement. Leg stiffness is calculated from maximum
vertical force and ‘leg spring’ compression (L). Joint stiffness is
calculated from changes in joint angles and moments
A. V. Lorimer, P. A. Hume
123
[18, 19]. Despite the simplicity of the model, the
mechanics of gait are efficiently represented. Centre of
mass displacement is achieved via compression of the ‘leg
spring’ whose length is modified via rotation of the joints
(Fig. 1a, b). The rate and extent of joint rotation is con-
trolled by the surrounding muscles, ligaments and tendons
working against the externally applied force. Control of
stiffness is likely to be both centrally mediated from the
cortex and possibly central pattern generators as well as
through reflex activation [20–22].
Hopping and running tasks are both used for stiffness
analysis. The task and biomechanical stiffness model used
are largely determined by the constraints of the testing
environment and equipment available. The basic equations
for estimating stiffness are:
kvertical ¼Fmax
Dy; ð1Þ
kleg ¼Fmax
DL; ð2Þ
kjoint ¼DMDh
; ð3Þ
where Fmax is the maximum vertical force, Dy is the centreof mass displacement, DL is the change in ‘leg spring’
length, DM is the change in joint moment and Dh is the
change in joint angle. Owing to the biarticular nature of the
gastrocnemius muscle there is potential for both the knee
and ankle to play a role in Achilles injuries.
There have been few studies investigating the link
between stiffness and Achilles tendon injuries. As Achilles
injuries are the result of cumulative damage and are a
progressive injury it is likely that a number of risk factors
come together in each individual to result in the injury.
Each risk factor by itself may be statistically insignificant.
The authors believe that joint and leg stiffness provide a
holistic view of how the lower limb is functioning during
running. Stiffness therefore could provide a measure that
was able to pick up risk by taking into account all the
different risk factors of the individual athlete. The limita-
tions regarding methods used to quantify stiffness, in par-
ticular the complex link between global measurements and
local alterations, means that associations with injury may
be difficult to determine.
2 Objective
Using the risk factors for Achilles injuries identified in our
previous review [12], vertical ground reaction force,
braking and propulsive force, surface stiffness and arch
height, the objective of this systematic review was to
determine whether there is a link between running related
risk factors for Achilles tendon injuries and lower
extremity stiffness. The findings will direct further research
into measures for identifying Achilles tendon injury risk.
3 Methods
Cochrane Collaboration review methodology (literature
search; assessment of study quality; data collection of
study characteristics; analysis and interpretation of results;
recommendations for clinical practice and further research)
was used [23].
3.1 Search Parameters and Criteria
Databases PubMed, SPORTDiscus, CINAHL and Web of
Science to October 2015 were searched for terms linked
with Boolean operators (‘‘AND’’, ‘‘OR’’, ‘‘NOT’’): verti-
cal/leg/joint/ankle/knee/hip stiffness; arch height; braking
force; vertical force; running distance; running experience;
eccentric strength; concentric strength; knee flexion; ankle
dorsiflexion; ankle eversion; ankle coupling; tibial rotation;
muscle activity; running speed; age; sex. Papers were
selected based on title, then abstract and finally text. Rel-
evant references from the text of selected articles were also
retrieved and included in the analysis. Papers were exclu-
ded if their content included the following topics: any
movement that was not running such as vertical drop jump;
tendon stiffness; musculoarticular stiffness; oscillation
stiffness method; passive stiffness; sled testing; change of
direction; stretching; upper body, without including run-
ning or hopping stiffness measures. Case reports, reviews,
editorials, letters to the editor and all animal studies were
excluded. Papers were included that specifically addressed
aspects of training or racing encountered by triathletes,
such as fatigue and running off the bike.
A total of 13,529 papers were identified of which 5762
were duplicates. After selection for inclusion criteria and
elimination based on exclusion criteria, 57 papers were left
for inclusion into the final review (Fig. 2).
3.2 Assessment of Study Quality for Systematic
Review and Data Extraction
Owing to the diversity of study types (between subject
repeated measure, within subject repeated measure and
correlation analysis as reported in Electronic Supplemen-
tary Material Appendix S1), and the known influences on
stiffness, a new nine-item study inclusion criteria scale was
developed based on the PEDro [24] and Bizzini scales [25]
(Table 1). Exclusion criteria as well as inclusion criteria
were deemed important as this gave better insight into the
characteristics of the groups being studied as the nature of
Stiffness as a Risk Factor for Achilles Tendon Injury
123
the research does not allow blinding and true randomisa-
tion. Body weight normalisation of results was important
when groups were significantly different owing to the
effect that body weight has on stiffness measures. Where
randomisation was not possible such as cross-over with
only one measurement, or a reason for lack of randomi-
sation was given, the study quality point was awarded.
Stiffness changes required two means, effect sizes or cor-
relations and required confidence intervals or standard
deviations to meet these criteria. The quality scores based
on the paper selection criteria ranged from 4 to 9.
3.3 Analysis and Interpretation of Results
All differences in stiffness between study conditions were
converted into effect sizes. Effect sizes with 95 % confidence
limits were calculated from means and standard deviations.
Fig. 2 Flow of information
through the different phases of
the systematic review.
k stiffness
Table 1 Study quality score criteria
Number Criteria
1 Eligibility criteria were specified (specifically activity
level)
2 Exclusion criteria described (specifically injury criteria)
3 Each group contained at least ten participants
4 Groups were similar at baseline for height, weight, sex and
age (no significant difference) or results were weight
adjusted
5 Randomisation was employed where necessary
6 Frequency and/or horizontal velocity were specified
7 A measure of change in stiffness and variability for at least
one key variable was given
8 At least five landings per person per condition were used
for stiffness analysis
9 Statistical analysis was detailed
A. V. Lorimer, P. A. Hume
123
Both between- and within-subject repeated-measure effect
sizes were calculated as the difference between the means
divided by the pooled standard deviation. It is acknowledged
that this method may overestimate the effect size and confi-
dence interval in within-subject analysis, owing to the lack of
complete independence; however, the lack of exact p values in
the majority of studies prevented the use of alternative
methods [26].Tomaintain consistency, all resultswere treated
with the same method regardless of reporting of p values.
Correlations were converted to effect sizes by d ¼ 2rffiffi
1p
�r2
(where d = effect size and r = correlation coefficient) with
confidence intervals calculated using the Fisher’s z transfor-
mation prior to effect-size conversion [23]. Study character-
istics and quality scores are reported in Electronic
Supplementary Material Appendix S1. Stride rate analysis
in a variety of running populations ranged from 1.3 to
1.7 Hz [27, 28]; therefore, frequency conditions were
limited to the range between 1.5 and 2.5 Hz (preferred
hopping frequency = *2.2 Hz). Stiffness differences
related to the factors previously identified as clearly
increasing or decreasing risk of Achilles tendon injuries
[12], are presented in Table 2 while all other risk factors
are presented in Table 3. An effect size of 0.2–0.6 was
considered small, 0.6–1.2 moderate, 1.2–2.0 large and
greater than 2.0 very large [29].
4 Results
In our previous review on risk factors for Achilles injuries
in running athletes, high peak braking force was shown to
increase Achilles tendon injury risk, while high peak
propulsive and vertical force, increasing surface stiffness
and increasing arch height were protective [12]. Two
studies investigated braking force and vertical and leg
stiffness [30, 31]. Increasing braking force showed a large
increase in vertical and leg stiffness when running at pre-
ferred pace [30]. However, when running at 95 % of
VO2max (maximum volume of oxygen uptake) there was no
clear effect [31] (Table 2). Increased propulsive force
(Fymax), which demonstrated small protective effects for
Achilles injuries [12], was associated with moderate to
large increases in both vertical and leg stiffness [30, 31].
Three studies reported the relationship between vertical
force (Fzmax) and vertical and leg stiffness [30, 32, 33].
Running at preferred pace showed clear increases in ver-
tical and leg stiffness with increasing force [30]. Increased
vertical force with sprinting was associated with an unclear
increase in vertical and an increase in leg stiffness [33].
Treadmill running at 80 % VO2peak (peak volume of oxy-
gen uptake), however, gave an increase in leg stiffness and
decrease in vertical stiffness, both of which had unclear
effect sizes [32].
As surface stiffness or arch height was increased, risk of
Achilles tendon injury was reduced [12]. Low arches and
low surface stiffness may be harmful to tendon health [12].
Three studies investigated changes in vertical stiffness [34–
36], six leg stiffness [34–39], and one knee and ankle
stiffness with changing surface stiffness [37]. There was an
increase in vertical stiffness when running across a surface
with increasing stiffness at 3.0 m/s [34]. All other results
for vertical stiffness were not clear and showed decreased
or unchanged stiffness. Of the seven comparisons that
showed clear results, all but one showed decreased leg
stiffness with increasing surface stiffness for both hopping
and running [34, 37, 39]. All results for knee and ankle
stiffness during bipedal hopping were unclear [37]. The
trend was for decreasing ankle stiffness and increasing
knee stiffness as surface stiffness increased. Increasing the
stiffness of the midsole resulted in small decreases in both
knee and ankle stiffness [40]. Only one study has compared
arch height and leg and joint stiffness [41]. Increasing arch
height was associated with an unclear but moderate
increase in leg stiffness and a moderate increase in knee
stiffness [41].
Other risk factors (pace variables, age, sex, footwear,
muscle activity patterns, intensity and rearfoot angle) did
not show distinct association with Achilles tendon injury
but could still be involved in the injury process [12]. The
triathlon-specific variable of transitioning from cycling to
running was also considered for injury risk potential. As
fatigue cannot be discounted from any cycle to run tran-
sition effects, the effects of fatigue was incorporated.
Results were variable with only increasing muscle activity
and decreasing contact time showing conclusive increased
joint stiffness (Table 3). The effect of these risk factors on
stiffness are given in Table 3 and the general trend of the
data summarised.
Velocity, and parameters associated with velocity, were
measured for all stiffness variables. An increase in velocity
was generally associated with moderate to large increases
in vertical stiffness [30, 33, 42–44]. Leg, knee and ankle
stiffness showed trivial to small increases with increasing
velocity [30, 33, 42, 43, 45]. Increasing contact time
resulted in decreased vertical [30, 33, 46], leg [30, 33, 47–
49] and ankle stiffness [45, 50, 51]. Increasing stride rate
resulted in increased vertical [30, 32, 33, 43, 52–54], leg
[30, 33, 43, 48, 50, 53–56], knee and ankle stiffness [50,
56]. Stride length however was variable, giving increases
[30, 33, 43], decreases [31, 33, 49] and no change [30, 43]
for both vertical and leg stiffness.
Bipedal hopping at 2.2 and 2.0 Hz resulted in male
individuals having moderately larger leg stiffness [57] and
moderately smaller leg stiffness [58] compared with female
individuals. At 2.5 Hz [58] and preferred hopping fre-
quency [59, 60], the differences were small and both
Stiffness as a Risk Factor for Achilles Tendon Injury
123
Table
2EffectofthefiveclearAchillestendonrisk
factorsonlower
bodystiffnessmeasures.Clear
results(CIdoes
notcross
zero
[29])areshaded
forclarity.Anegativeeffectsize
indicates
that
decreasingthevariable
ofinterest
orthefirstconditionstated
resulted
inalower
stiffnessmeasure
Factor
Change
Movem
ent
Vertical
Leg
Knee
Ankle
Reference
ES
95%
CI
ES
95%
CI
ES
95%
CI
ES
95%
CI
Fymin
Correlation
Run(preferred)
1.57
0.14,3.70
1.07
-0.28,2.90
[30]
Correlation
Run(95%
VO2max)
-0.06
-1.86,1.70
[31]
Fymax
Correlation
Run(preferred)
1.79
0.30,4.06
0.81
-0.52,2.50
[30]
Correlation
Run(95%
VO2max)
1.91
0.10,5.00
[31]
Fzm
ax
Correlation
Sprint(m
ax)
0.87
-0.24,2.25
1.42
0.24,3.04
[33]
Correlation
TM
run(80%
VO2peak)
-0.45
-1.75,0.70
0.95
-0.21,2.43
[32]
Correlation
Run(preferred)
1.85
0.35,4.18
2.98
1.12,6.15
[30]
Surfacek
21.3–533kN/m
Run(3.0
m/s)
2.27
0.90,3.64
-2.88
-4.48,-1.28
[34]
Cont.21.3–cont.533kN/m
0.46
-1.14,2.07
-3.58
-5.07,-2.09
220–950kN/m
Run(3.7
m/s)
-0.56
-3.42,2.29
0.23
-4.15,4.60
[36]
450–950kN/m
-0.37
-2.83,2.10
0.11
-4.03,4.25
75–950kN/m
-1.22
-5.05,2.61
0.78
-3.31,4.87
Low–high
Run(5.0
m/s)
0.04
-6.99,7.07
-1.14
-5.40,3.12
[35]
30–35,000kN/m
Bihop(2.2
Hz)
-12.86
-13.86,-11.87
3.09
-117.07,123.24
-3.62
-47.56,40.31
[37]
60.9–35,000kN/m
-7.95
-10.33,-5.57
0.07
-95.41,95.55
-0.06
-36.77,36.65
30–60.9
kN/m
2.47
0.02,4.93
2.69
-132.42,137.80
-3.05
-54.54,48.45
26.1–50.1
kN/m
Bihop(2.0
Hz)
-2.29
-19.84,15.27
[38]
27–411kN/m
(expected)
Bihop(2.2
Hz)
-5.15
-6.88,-3.42
[39]
27–411kN/m
(surprise)
-6.00
-7.63,-4.38
Cont.27–cont.411kN/m
-8.57
-10.14,-7.01
Archheight
Low
arch–higharch
Run(3.4
m/s)
0.65
-0.07,1.38
0.63
0.61,0.65
[41]
Allcleareffects(CIs
donotcross
zero)areitalics
bihopbipedal
hopping,CI95%
confidence
interval,cont.continuousstiffnesssurface,
ESeffect
size,F-ymaxmaxim
um
propulsiveforce,
Fyminmaxim
um
brakingforce,
Fzm
axmaxim
um
verticalforce,
maxmaxim
um
effort,TM
treadmill,VO2maxmaxim
um
oxygen
uptake,
VO2peakpeakoxygen
uptake
A. V. Lorimer, P. A. Hume
123
Table
3EffectofAchillestendoninjury
risk
factors
onlower
bodystiffnessmeasures.Clear
results(CIdoes
notcross
zero
[29])areshaded
forclarity.A
negativeeffect
size
indicates
that
decreasingthevariable
ofinterest
orthefirstconditionstated
resulted
inalower
stiffnessmeasure
Factor
Change
Movem
ent
Vertical
Leg
Knee
Ankle
Reference
ES
95%
CI
ES
95%
CI
ES
95%
CI
ES
95%
CI
Velocity
Correlation
Sprint(m
ax)
0.80
-0.31,2.15
0.63
-0.47,1.92
[33]
Correlation
Sprint(m
ax)
1.84
-0.11,5.29
0.18
-1.74,2.25
[43]
Correlation100m
Sprint
1.28
-0.55,4.17
[44]
70%
(7.0
m/s)–80%
(7.8
m/
s)
Sprint
0.91
-33.66,35.49
-0.10
-4.37,4.17
0.00
-1.49,1.49
[45]
80–90%
(8.8
m/s)
0.59
-53.21,54.39
0.33
-4.76,5.42
0.00
-1.74,1.74
90–100%
(9.7
m/s)
0.04
-58.39,58.46
0.51
-10.96,11.99
0.20
-1.91,2.31
70–100%
1.58
-39.83,42.99
0.64
-10.48,11.77
0.22
-1.69,2.13
Correlation
Run(preferred)
1.73
0.26,3.97
0.15
-1.23,1.58
[30]
2.5–3.5
m/s
Run
0.71
-7.32,8.74
-0.15
-3.33,3.02
0.76
-1.85,3.37
0.50
-2.52,3.53
[42]
3.5–4.5
m/s
1.41
-10.30,13.12
0.75
-2.24,3.74
0.85
-2.76,4.45
0.62
-2.94,4.17
4.5–5.5
m/s
0.80
-10.66,12.25
0.00
-2.08,2.08
0.23
-3.55,4.00
-0.02
-3.68,3.65
3.0–4.0
m/s
TM
run
0.10
-2.57,2.77
[122]
Contact
time
Correlation
Sprint(m
ax)
-0.80
-2.15,0.31
-1.06
-2.52,0.07
[33]
Correlation
Run(preferred)
-1.79
-4.06,-0.30
-1.70
-3.92,-0.24
[30]
Correlation
Run(vVO2max)
-6.60
-20.87,-1.72
[49]
Short-preferred
Run(3.3
m/s)
-2.49
-5.37,0.39
[48]
Preferred-long
-3.44
-5.72,-1.17
Short-preferred
Bihop(preferred)
-1.14
-9.73,7.44
[47]
Eld.correlation
Bihop
-1.54
-2.80,-0.57
[51]
70%
max
correlation
Sprint
-2.76
-6.32,-0.79
100%
max
correlation
-4.69
-10.18,-1.91
SR/Hz
Correlation
Sprint(m
ax)
2.08
0.74,4.03
0.90
-0.22,2.29
[33]
Correlation
Sprint(m
ax)
1.67
-0.23,4.94
0.29
-1.60,2.42
[43]
Correlation
TM
run(80%
VO2peak)
3.23
1.49,6.02
[32]
Correlation
Run(preferred)
2.01
0.47,4.44
0.24
-1.11,1.71
[30]
-8%—
preferred
Run(78%
VO2max)
0.95
-2.22,4.12
0.55
0.46,0.64
[75]
Preferred—
?8%
1.28
-2.06,4.61
2.43
2.38,2.48
-26%—
preferred
TM
run(2.5
m/s)
8.85
5.62,12.09
3.09
0.10,6.07
[53]
Preferred—
?30%
5.69
-0.78,12.16
2.53
-0.70,5.76
-30%—
preferred
Run(3.3
m/s)
0.44
-2.30,3.18
[48]
Preferred—
?30%
1.43
-4.99,7.86
1.5–2.1
Hz
Bihop
0.72
0.48,0.96
0.52
0.28,0.76
0.65
0.52,0.77
[50]
Stiffness as a Risk Factor for Achilles Tendon Injury
123
Table
3continued
Factor
Change
Movem
ent
Vertical
Leg
Knee
Ankle
Reference
ES
95%
CI
ES
95%
CI
ES
95%
CI
ES
95%
CI
1.5–2.1
Hz(/kg)
Bihop
1.13
1.00,1.26
0.55
-10.23,11.33
0.80
-2.48,4.09
[56]
1.5–2.2
Hz(D
)Unihop
2.43
2.38,2.48
[55]
1.5–2.2
Hz(N
D)
1.75
1.71,1.79
-20%—
preferred
Unihop
0.90
-5.48,7.28
[52]
Preferred—
?20%
1.23
-5.45,7.90
Stridelength
Correlation
Sprint(m
ax)
-0.61
-1.89,0.49
-0.56
-1.83,0.54
[33]
Correlation
Sprint(m
ax)
0.59
-1.24,2.90
0.00
-1.99,1.98
[43]
Correlation
Run(95%
VO2max)
-0.39
-2.33,1.29
[31]
Correlation
Run(preferred)
0.75
-0.58,2.41
-0.07
-1.49,1.31
[30]
Correlation
Run(vVO2max)
-4.06
-13.23,-0.66
[49]
Age
Men–boys
Bihop(1.5
Hz)
-0.79
-7.03,5.45
[61]
Young–eld.(50%
max)
Bihop
0.19
-2.38,2.75
0.34
-0.95,1.62
[51]
Young–eld.(75%
max)
0.64
-3.60,4.87
0.00
-1.53,1.53
Young–eld.(100%
max)
-0.06
-4.24,4.11
-1.51
-2.96,-0.06
16–20–36–60y
Run(3.3
m/s)
0.00
-0.10,0.10
-0.47
-0.69,-0.24
[40]
\35–50?
yRun(3.8–4.1
m/s)
0.74
-13.63,15.11
-0.50
-0.97,-0.03
0.71
-0.37,1.78
[63]
Young–eld.
Bihop(2.2
Hz)
-0.01
-0.10,0.07
[62]
Sex
Fem
ale–
male
Bihop(2.0
Hz)
-0.72
-0.79,-0.65
[58]
Fem
ale–male
Bihop(2.2
Hz)
0.68
0.57,0.79
[57]
Fem
ale–male
Bihop(2.5
Hz)
-0.15
-0.28,-0.02
[58]
Fem
ale–male
Bihop(preferred)
0.23
0.10,0.36
[102]
Fem
ale–male
Bihop(preferred)
-0.06
-0.23,0.11
[60]
Fem
ale–male
Bihop(preferred)
0.36
0.27,0.44
[59]
Fem
ale35?
y–male35?
yRun(3.3
m/s)
1.70
1.55,1.84
0.19
-0.08,0.46
[40]
Footwear
Barefoot–350-g
shoe
TM
run(3.6
m/s)
-0.69
-4.06,2.68
-0.68
-2.13,0.77
[64]
Shoe-racingflat
(fem
ale)
Run
0.22
-30.39,30.82
[68]
Shoe-racingflat
(male)
0.90
-45.45,47.25
Barefoot–shoe
Hop(2.2
Hz)
4.00
3.30,4.69
[66]
Barefoot–shoe(0-m
m
midsole)
Run(3.3
m/s)
-0.08
-3.06,2.90
0.26
-17.6,18.12
1.07
-31.8,33.95
[67]
0mm
mid–16-m
mmidsole
Run(3.3
m/s)
-0.23
-2.98,2.52
-0.46
-19.2,18.28
0.07
-35.74,35.87
[67]
Soft–hardmidsole
Run(3.3
m/s)
-0.26
-0.32,-0.20
-0.48
-0.62,-0.34
[40]
Minim
al–standardshoe
TMRun(65%
VO2max)
-0.56
-2.85,1.72
-0.53
-1.99,0.94
[65]
A. V. Lorimer, P. A. Hume
123
Table
3continued
Factor
Change
Movem
ent
Vertical
Leg
Knee
Ankle
Reference
ES
95%
CI
ES
95%
CI
ES
95%
CI
ES
95%
CI
Training
status
Untrained–endurance
Bihop
1.64
1.52,1.75
1.63
-5.66,8.92
-4.06
-5.90,-2.22
[71]
RFA
Runcorrelation
Run(3.4–4.2
m/s)
-1.06
-1.98,-0.29
[123]
Staticcorrelation
-0.93
-1.81,-0.17
Closed–opened
-1.13
-3.00,0.74
Intensity
3–4BW
Bihop
0.08
-8.43,8.60
1.96
-2.46,6.38
1.00
-0.63,2.63
[69]
3–6BW
0.87
-6.57,8.32
7.07
3.42,10.71
2.33
1.11,3.56
Strength
Pre–post-w
eightedsled
train
Run(3.3
m/s)
0.42
-24.67,25.51
[72]
Muscle
activity
Gpreact.correlation
Run(preferred)
0.67
-0.98,2.76
1.15
-0.51,3.58
[70]
VLonsetcorrelation
Run(3.4
m/s)
-0.65
-1.77,0.31
[41]
Solpreact.correlation
Bihop
0.90
0.19,1.71
1.06
0.34,1.92
[69]
Lat.G
preact.correlation
1.09
0.37,1.95
0.70
0.00,1.48
Med.G
preact.correlation
1.06
0.34,1.92
Eld.sol(BR/PR)correlation.
Bihop
1.67
0.67,2.97
[51]
Eld.med.G(BR/PR)
correlation
2.14
1.04,3.62
Eld.lat.G
(BR/PR)
correlation
2.41
1.26,4.02
Eld.TA
(BR/PR)correlation
-1.71
-3.03,-0.71
Eld.TA/solcoact.correlation
-1.09
-2.20,-0.19
Transition
CR–5%
T2
Run(LT)
0.37
-3.58,4.32
0.59
-1.28,2.46
[88]
CR–20%
T2
0.20
-3.68,4.07
0.35
-1.45,2.15
CR–100%
T2
0.08
-3.63,3.78
0.20
-1.58,1.98
Fatigue
2nd–12th
sprint
Sprint(m
ax)
-4.69
-7.40,-1.98
-1.73
-2.25,-1.20
[33]
25–375m
Sprint(m
ax)
-3.26
-15.53,9.01
-1.88
-5.68,1.92
[43]
Sprint1–sprint2
Sprint
-0.71
-9.61,8.18
-0.28
-5.71,5.15
[73]
Sprint1–sprint4
-0.39
-8.06,7.29
1.13
-3.18,5.44
1st–2nd100m
Sprint
-1.11
-14.43,12.21
-0.62
-5.20,3.96
[44]
1st–4th
100m
-1.43
-17.19,14.34
-0.24
-6.08,5.59
10–100%
Run(V
O2max)
-0.02
-5.10,5.06
-0.42
-3.21,2.37
[83]
10–100%
Run(95%
VO2max)
0.09
-2.51,0.79
-0.86
-2.51,0.79
[31]
Pre–post
TM
run(80%
VO2peak)
-0.27
-2.62,2.09
-0.26
-1.16,0.64
[32]
Pre–post
Run(78%
VO2max)
-0.17
-4.13,3.79
-0.10
-1.61,1.41
[75]
Stiffness as a Risk Factor for Achilles Tendon Injury
123
Table
3continued
Factor
Change
Movem
ent
Vertical
Leg
Knee
Ankle
Reference
ES
95%
CI
ES
95%
CI
ES
95%
CI
ES
95%
CI
Pre–post
Run(70%
VO2max)
1.30
0.94,1.66
0.42
-0.01,0.85
[76]
Pre–post
?7-m
inmax
2.82
2.54,3.11
0.28
-0.03,0.59
Fatigue
0–2h
TM
run(2.8
m/s)
0.01
-5.60,5.63
0.03
-3.69,3.75
[78]
0–4h
0.31
-5.01,5.63
0.20
-3.31,3.72
Pre–post-50min
TM
run(65%
VO2max)
0.10
-2.62,2.42
-0.53
-1.99,0.94
[65]
Pre–post-exhaustion
Run(V
O2max)
-0.35
-25.77,25.08
-0.89
-2.72,0.95
[49]
Pre–post-exhaustion
Run(95%
VO2max)
0.13
-10.27,10.53
-0.51
-2.38,1.36
[84]
Pre–post-M
tmarathon
Run(3.5–3.7
m/s)
-0.67
-5.28,3.94
-0.57
-1.83,0.70
[85]
Pre–post
uphillmarathon
Run(2.9–2.7
m/s)
-3.90
-6.20,-1.59
-3.68
-4.55,-2.82
[86]
Pre–post-m
ountain
ultra
Run(3.3
m/s)
0.47
-2.33,3.25
-0.14
-1.09,0.82
[80]
Pre–post-m
ulti-stagerace
Run(3.9
m/s)
0.43
-5.07,5.93
[81]
Pre–post-cyclesprints
Run(2.8
m/s)
0.08
-3.39,3.55
0.00
-3.19,3.19
Pre-post
TM
run(3.3
m/s)
0.77
-5.16,6.70
0.83
-0.60,2.26
[74]
Pre–3hpost
Run(3.3
m/s)
0.52
-1.49,2.54
-0.33
-1.24,0.57
[77]
Pre–post
Run(3.6
m/s)
0.01
-6.48,6.50
0.01
-2.84,2.86
[87]
1st–4th
lap
Run(race)
-0.70
-2.12,0.72
-0.34
-1.33,0.65
[82]
1st
lap–finishchute
-0.09
-1.71,1.54
-0.12
-1.19,0.95
200–1000m
Run(preferred)
0.31
-0.11,0.73
0.52
0.11,0.93
[30]
200–2000m
-1.73
-2.11,-1.35
-0.03
-0.42,0.35
200–5000m
-1.36
-2.08,-0.65
-0.06
-0.49,0.37
Pre-post
squats(m
ale)
Bihop(preferred)
0.24
0.08,0.40
0.18
-17.43,17.78
[60]
Pre-post
squats(fem
ale)
0.16
-6.78,6.97
0.18
-11.28,11.65
Allcleareffects(CIs
donotcross
zero)areitalic
bihopbipedal
hopping,BRbraking,BW
bodyweight,CI95%
confidence
interval,coact.Coactivation,CRcontrolrun,D
dominantkickingleg,Eld.Elderly,ESeffect
size,lat.G.lateral
gastrocnem
ius,LTlactatethreshold,med.G.medialgastrocnem
ius,maxmaxim
um
effort,NDnon-dominantkickingleg,postpost-runningexercise,PRpropulsive,preact.Preactivation,pref.
preferred,pre
priorto
runningexercise,RFArear-to-forefootangle,solsoleus,SRstriderate,T2transitionrun,TAtibialisanterior,TM
treadmill,unihopunipedalhopping,VLvastuslateralis,
vVO2maxvelocity
atVO2max,VO2maxmaxim
um
oxygen
uptake,
VO2peakpeakoxygen
uptake
A. V. Lorimer, P. A. Hume
123
positive and negative. For overground running, men over
35 years of age showed higher knee stiffness but similar
ankle stiffness to women over the age of 35 years [40].
Men had greater leg stiffness than boys [61]. Compared
with young adults, active older individuals had greater knee
and ankle stiffness, except when hopping at 100 % of
maximum hopping height [51]. Leg stiffness exhibited no
change [62]. Age above 35 years may result in increased
vertical stiffness when running [63], a decrease [63] or no
change [40] in knee stiffness and either an increase [63] or
decrease [40] in ankle stiffness. Wearing shoes caused
decreased vertical and leg stiffness compared with running
barefoot [64]. Minimalist shoes gave similar results to
barefoot running when compared with standard shoes [65].
When hopping, wearing shoes caused an increase in ver-
tical and leg stiffness [66]. However, when comparing
barefoot running and shoes with a 0-mm pitch leg stiffness
was unchanged while knee and ankle stiffness showed a
small and moderate increase, respectively [67]. Racing flats
caused vertical stiffness to increase slightly compared with
standard shoes [68]. Increased triceps surae activity was
associated with increased leg, knee and ankle stiffness [51,
69, 70]. Tibialis anterior activity was positively correlated
to decreased ankle stiffness [51]. Endurance training gave
greater leg and knee stiffness but reduced ankle stiffness
compared with untrained individuals [71]. Increasing
hopping intensity based on body weight resulted in small to
moderate increases in leg stiffness but larger increases in
knee and ankle stiffness [69]. No direct relationship
between concentric strength and stiffness have been
reported however, a resistance training program that
reported an average of 20 % improvement in one repetition
maximum for half squat, showed a small increase in ver-
tical stiffness with very large confidence intervals [72].
Fatigue was induced by a large variety of methods and
was associated with approximately equal numbers of
increased [30, 31, 44, 73–81], decreased [30–33, 43, 44, 49,
65, 73, 82–86] and unchanged [30, 31, 49, 60, 65, 75–80,
82–84, 87] stiffness for both vertical and leg stiffness.
Transitioning from cycling to running resulted in small
increases in vertical and leg stiffness compared with run-
ning alone [88].
Figure 3 provides a summary of the links between risk
factors for Achilles tendon injuries and measures of lower
leg stiffness.
5 Discussion
Summarising the variables that alter stiffness is a difficult
task because of the multifactorial nature of the measure-
ment. Both hopping and running have been used to deter-
mine the effect of different variables on stiffness measures.
While hopping is a cyclic motion with similar vertical
centre of mass motion, the addition of horizontal force and
motion adds additional information to the ‘spring-mass
model’. Hopping is often used as a surrogate for running;
however, whether it is a valid surrogate when investigating
stiffness is unknown. Small sample sizes and large confi-
dence intervals resulted in effect sizes spanning a wide
range of interpretations. Combining effect sizes was diffi-
cult owing to each study using different parameters of
change to the variable of interest.
5.1 Link Between Stiffness and Clear Achilles
Injury Risk Factors
In our previous review on risk factors for Achilles injuries,
high braking force was shown to have a clear detrimental
effect on Achilles tendon health [12]. Increasing surface
stiffness, arch height and propulsive and vertical forces
were protective [12]. Running on soft surfaces and having a
low arch were therefore interpreted as harmful to the
Achilles tendon.
High braking forces were clearly associated with high
vertical stiffness. Leg stiffness was also increased with
increasing braking force at preferred running pace. A low
surface stiffness caused increased leg and ankle stiffness
compared with surfaces of higher stiffness. The greater the
change in surface stiffness the more distinct the change in
stiffness. Large confidence intervals for the joints meant
the effect was unclear. It is possible that while the ankle
stiffness is increased the knee stiffness is decreased in
response to low surface stiffness. Low arch height was
clearly related to decreased knee stiffness and showed a
negative association with leg stiffness. Therefore, an
increase in lower body stiffness associated with high
braking forces or low surface stiffness could be related to
Achilles tendon injuries. However, an increase in knee and
leg stiffness when associated with high arches is potentially
protective against Achilles tendon injuries. Increased
propulsive force and increased vertical force were also
protective for Achilles tendons, yet these were also asso-
ciated with increases in vertical and leg stiffness. Further
prospective research is needed to verify a link between
increased stiffness and risk of Achilles injury.
The mechanical properties of the Achilles tendon con-
tribute to only a part of the lower limb stiffness that
includes the musculo-articular stiffness of the hip, knee and
ankle joints. Runners may exhibit significant variations in
lower limb stiffness because of alterations at the patella
tendon or fatigue in knee extensor muscles with unknown
consequences for the risk of Achilles tendon injury. Joint
stiffness does not reflect the intrinsic mechanical properties
of the Achilles tendon as it is affected by a wide range of
factors: neuromuscular coordination, mechanical properties
Stiffness as a Risk Factor for Achilles Tendon Injury
123
of muscular and non-muscular structures (skin, ligaments,
aponeuroses, tendon; [89, 90]). Validated procedures exist
to assess the mechanical properties of the tendon (see
Seynnes et al. [91] for a review); therefore, prospective
studies could be conducted to evaluate the impact of
Achilles tendon stiffness on the risk of Achilles tendon
injury.
5.2 Link Between Other Achilles Injury Risk
Factors and Stiffness
Both protective and injurious factors appear in general to
cause increases in stiffness variables. However, many other
risk factors have been identified to be potentially related to
Achilles injury risk [12]. Overuse injuries are characterised
by slow progressive onset, suggesting that the injury is the
result of accumulation of damage from low levels of
overload. Therefore, the changes to movement patterns
would be expected to be small. Combined with the high
variability resulting from looking at individual aspects of a
movement (e.g., eversion angle in foot pronation), it is
probable that risk factors will give inconclusive results
when using a single-risk factor analysis approach. There-
fore, the effect of other Achilles injury risk factors on
lower body stiffness variables was also considered.
5.2.1 Velocity or Running Pace
Pace is a commonly cited risk factor [92] for overuse
injuries in general. Faster running pace, or increasing pace
Fig. 3 Forest plot summary of evidence for the association between
Achilles injury and lower extremity stiffness variables. Black
diamonds show the effect size and confidence intervals for the
variable of interest with respect to Achilles injury risk. Grey dots
represent the effect size and associated confidence intervals for the
variable of interest with respect to the effect on stiffness. k stiffness, Dchange, vert vertical
A. V. Lorimer, P. A. Hume
123
too rapidly, are both thought to be associated with injury
[46, 92–94]. Vertical stiffness showed an increase in
stiffness with increasing velocity; however, the majority of
results were unclear. Decreased contact time and increased
stride rate (which are inherently related to velocity) caused
varying magnitudes of vertical stiffness increase. Increas-
ing stride length was associated with both increases and
decreases in vertical stiffness suggesting little to no effect
of stride length on vertical stiffness.
Leg and joint stiffness were much less conclusive. The
number of small to moderate increases in leg stiffness and
no change in leg stiffness with increasing velocity were
approximately equal. Variations in the methods used to
estimate changes in the length of the ‘leg spring’ were most
likely responsible for this variation in outcomes. Arampatzis
et al. [42] investigated changing overground running
velocity using the original McMahon and Cheng model
(Fig. 1a) [19] and found no change with increasing velocity.
However when change in leg length was measured from
centre of pressure to centre of mass, rather than estimated,
then leg stiffness increased with velocity [42]. Decreasing
contact time and increasing stride rate were associated with
small to very large increase in leg stiffness, while increasing
stride length tended to have little impact on leg stiffness. In
the McMahon and Cheng model (Fig. 1; Eqs. 1 and 2), the
difference between vertical and leg stiffness resulted from
the addition of horizontal motion. Center of mass trajectory
is unlikely to follow a perfect curve; therefore, direct mea-
surement is more likely to detect changes in leg length and
therefore leg stiffness than the estimated model. Direct
measurement may also be more effective at detecting
changes in leg length that are the result of overstriding.
Factors that modify stride length, such as flight time and
angle of attack, occur prior to contact. However, the distance
from the centre of pressure to the centre of mass can affect
the trajectory of the centre of mass. Therefore, how changes
in stride length alter stiffness is dependent on whether the
changes in stride length occur during flight or initial stance
and the method of measuring stiffness. This could account
for the variability in results shown. Contact time, however,
is dependent on changes in gait control occurring following
ground contact and therefore showed clearer associations
with stiffness.
Joint stiffness changes have only been investigated in
two studies [42, 45]. Increases in running speed were
associated with small to moderate increases in ankle stiff-
ness. Confidence intervals were very large for these mea-
sures. At the fastest running speeds (4.5–5.5 m/s) and
during sprinting, ankle stiffness appeared not to change.
Knee stiffness also showed smaller effect sizes for these
velocity changes. Contact time and contact rate showed the
same trends as vertical and leg stiffness, increasing stiff-
ness with changes related to increased velocity.
Only one of the effect sizes for increasing velocity was
clear, yet the changes to stiffness with stride rate and
contact time tended to be more conclusive. Stride length
increases with speed while contact time decreases. How-
ever, the exact relationship between speed and stride length
or contact time is a function of leg length [95]. Therefore,
different individuals will adjust running speed through
different combinations of altered contact time and stride
length. Such individuality in response would account for
the greater outcome variability seen when changing
velocity compared with altering contact time or stride rate.
Different combinations of increased stride rate with
decreased contact time, increased stride length, increased
propulsive force and increased flight time can all be used to
produce the same end running speed.
5.2.2 Exercise Intensity
Velocity and intensity are closely associated, with
increasing effort and force production required to run at
greater pace. Intensity was modified based on force pro-
duced normalised to body weight during hopping with a
greater target force associated with increased intensity.
Ankle, knee and leg stiffness were increased with increased
intensity, with greater increases in intensity resulting in
larger increases in stiffness. Loading of the Achilles tendon
during the braking phase of stance is eccentric with the
lengthening of the tendon controlled by action of the tri-
ceps surae muscle complex. During the propulsive phase,
loading is concentric in nature. Increasing the intensity of
the cyclic movement requires greater propulsive muscle
force, which is protective against Achilles injury. However,
greater loading during landing/braking can be injurious to
the Achilles tendon. Muscles and tendons work together to
dissipate energy during landing [96]. Tendons protect the
muscle during landing by allowing slower eccentric
actions. However, for the tendon to lengthen under load,
simultaneous contraction of the muscle is required [96].
5.2.3 Muscle Activity
The results clearly showed an increase in knee and ankle
stiffness with increasing triceps surae muscle activity and
overall leg stiffness. Greater braking/propulsive activity
ratio resulted in increased stiffness for all three triceps
surae muscles. Increasing muscle force increases the
stretch of the tendon [96]; therefore, a higher braking
activity and associated increased stiffness may cause
excessive tendon stretch. Achilles injury risk was associ-
ated more with timing of muscle activation between the
three muscles of the triceps surae rather than the level of
activity [97–100]. Fascicles of the Achilles tendon are
supplied by all three muscles, soleus and medial and lateral
Stiffness as a Risk Factor for Achilles Tendon Injury
123
gastrocnemius. Therefore, uneven activation could lead to
fascicles stretching at different rates and to different
lengths causing shear forces and consequent microscopic
ruptures within the tendon. Gastrocnemius braking/
propulsive ratio had a greater effect on stiffness than the
soleus, which may also influence the distribution of the
load within the tendon. Increased tibialis anterior activity
decreased stiffness as did greater tibialis anterior/soleus
activity. Increasing tibialis anterior activity may therefore
reduce the rate of eccentric loading, protecting the Achilles
tendon without sacrificing the triceps surae muscles.
5.2.4 Sex
Male individuals are reported to be more at risk of Achilles
injury than female individuals when over the age of
35 years [101]. The effect of sex on stiffness has only been
assessed in hopping studies. Both higher and lower stiff-
ness was observed in males when frequency was controlled
[57, 58]; however, there were only trivial to small differ-
ences between the sexes when allowed to adopt an indi-
vidual hopping frequency [59, 60, 102]. It is possible that
during running there would be little difference between
sexes, when running at their preferred stride rate. Increas-
ing age was associated with small or trivial increases in leg
and joint stiffness but was dependent on the method of
measuring stiffness. At 75 % of maximal hopping height,
ankle stiffness was similar but young people had lower
knee stiffness. At maximal hopping height, however, knee
stiffness was similar while ankle stiffness was lower in the
older individuals. This effect is probably owing to differ-
ences in muscle strength and tendon mechanical properties
which may decrease with age [103] and compensatory
movement patterns developed by active older individuals.
5.2.5 Footwear
Footwear is an environmental constraint to the gait task
similar to surface stiffness, where a surface of varying
stiffness is attached to the foot. The addition of shoes, or
changing from racing flats (low cushioning) to standard
shoes (high cushioning), however, caused decreased vertical
and leg stiffness during running. This effect is opposite to
changes in surface stiffness; however, it must be noted that
the effect sizes were small and confidence intervals large.
The opposite effect can be inferred from a comparison of
barefoot running and shod running, which found reduced
vertical ground reaction forces when barefoot [104]. It was
observed that runners adopted a midfoot strike pattern when
runners were barefoot, which resulted in greater ankle
stiffness but allowed reduced impact loading [104]. The foot
strike pattern rather than the shoe characteristics may be
more important in modifying stiffness.
5.2.6 Training Distance
Training distance is another common overuse injury risk
factor, but its role in Achilles tendon injuries was unclear
[12]. Training distance effect on stiffness has not been
measured, but leg and knee stiffness are greater in
endurance-trained athletes compared with untrained
individuals. Ankle stiffness was notably reduced for the
athlete group. However, this is a poor surrogate for
training distance as many other differences are apparent
between untrained and trained individuals including
motivation (training through pain), gait movement skill,
muscle strength and tendon loading, which all could
influence tendon health.
5.2.7 Pronation
Pronation may or may not be associated with Achilles
tendon injuries with the results varying depending on what
part of the movement was investigated [12]. The rate of
pronation and coupling with tibial rotation may be asso-
ciated with injury but the evidence is limited [12]. Prona-
tion, measured as rear to forefoot angle was shown to be
associated with stiffness during running for both static and
dynamic measurements. Increasing the angle (pronation)
resulted in decreased leg stiffness. Reducing normal
pronation increases impact loading [105]; therefore,
increased pronation is likely a protective adaptation. Pro-
prioceptive feedback is important in tuning muscle control
and movement patterns to achieve the desired task [106].
Therefore, the level of pronation is likely an adaptation
associated with muscle strength and forces unique to the
task and individual rather than a direct causative factor for
injury.
5.2.8 Triathlon-Specific Risk Factors
In triathlon, not only are the purely running related risk
factors important, but also factors associated with com-
bining three disciplines in the one race or training session.
Transitioning from cycling to running can result in feelings
of reduced coordination and heavy legs [107]. This period
of transition between the two disciplines is believed to
increase injury risk. Running economy [107–110] is
reduced in triathletes following cycling compared with
isolated running. Kinematic changes have been reported
following cycling [108, 111]. Others have reported no
changes in kinematics but altered muscle activity in some
athletes [112–114]. Proprioceptive feedback adaptations
for postural control persist for a short period following
cessation of running or cycling activity in triathletes [115].
Loss of proprioceptive feedback has been shown to affect
interjoint coordination [116]. However, the effect of
A. V. Lorimer, P. A. Hume
123
fatigue cannot be isolated from coordination effects when
assessing gait changes following cycling.
Fatigue was induced in a variety of ways. Repeated
sprints tended to cause reduced vertical and leg stiffness
[33, 44, 73], while sustained running at more aerobic
intensities (i.e.,\75 % VO2max) caused increased stiffness
depending on the intensity and the time [76–78, 87]. There
may be no effect of fatigue on lower body stiffness or
changes may be race distance specific. Shorter distance
athletes may experience decreased leg stiffness as fatigue
from high-intensity running develops. Longer distance
athletes may experience increases in stiffness as a race or
training session progresses. Fatigue induced by cycling
sprints had no effect on stiffness [79].
The effect of transitioning from cycling to running has
only been directly measured in one study [88]. Vertical and
leg stiffness showed small, unclear increases in leg and
vertical stiffness when running at lactate threshold. Chan-
ges in stiffness were largest immediately following cycling
and decreased as the run progressed. During an Interna-
tional Triathlon Union Championship race, stiffness
decreased throughout the run but athletes were able to
increase stiffness for the sprint down the finish chute. The
use of continuous drafting compared with alternative
drafting and non-drafting resulted in reduced stride length
and increased stride rate. Drafting reduces the energy cost
of cycling [117]. Therefore, gait changes following cycling
are more likely the result of muscular fatigue than coor-
dination changes.
In risk-factor analysis, looking at how individual risk
factors impact stiffness may not give the whole picture.
Related variables may have very different effects on stiff-
ness, as could be seen with velocity, stride rate and stride
length. Humans have high levels of variability in their
movement as a result of variable anthropometry, training,
skill and learning processes and therefore changes to
movement patterns in response to changing task constraints
are likely to be variable as well. Low stiffness has been
suggested to be associated with increased risk of soft-tissue
injuries [118]. However, this analysis suggested there is
potential for increased stiffness to be detrimental to tendon
health. Whether high stiffness can be used to predict
Achilles tendon injuries during running needs to be
investigated further in prospective studies.
Coaches and clinicians should be aware that increased
lower body stiffness during running may be associated with
Achilles tendon injury risk. Athletes returning from injury
should be advised to limit activities that result in increased
stiffness. Activities associated with increased stiffness
include high-intensity/speed running and running on soft
surfaces (i.e., sand or track). Using a wide pace range that
encourages variation in gait parameters may also assist in
distributing Achilles tendon loading. Pronation appears not
to be associated with this increased stiffness and therefore
correcting pronation with posted shoes or orthotics should
be viewed with caution as this may increase stiffness and
therefore injury risk in athletes. Gait retraining to reduce
braking forces or increase knee and ankle rotation during
stance may be beneficial for some athletes. However,
reducing stiffness may be detrimental to performance.
6 Conclusions
There have been few studies examining the link between
Achilles tendon injuries and lower extremity stiffness.
Those that have, included stiffness as part of a screen of
multiple biomechanical factors rather than looking at
stiffness as a holistic measure that summarises how the
limb or joint is functioning as a unit [119–121]. There is a
potential link between the various measures of stiffness and
risk of developing an Achilles tendon injury, which needs
to be investigated further with studies designed to specifi-
cally address this question. All joints need to be assessed
together to understand how they work together to create the
healthy or injury-promoting environment.
The review aimed to identify whether there was a link
between previously identified risk factors for Achilles
tendon injuries and measures of lower leg stiffness. Com-
bining the results to give an overview of the link between
different Achilles tendon injury risk factors and stiffness
was made difficult by the diversity of methods, models and
magnitude of variable changes used. We cannot say con-
clusively that factors that increase the risk of Achilles
tendon injuries also increase lower body stiffness; how-
ever, the trend of the results appears to suggest this.
Increased propulsive and vertical force, and increased arch
height, while protective for Achilles tendon injuries, also
increased stiffness levels. Other factors (e.g., pronation)
widely believed to be involved in increased risk of Achilles
tendon injuries, were associated with decreased stiffness
during running. Further longitudinal investigation into
whether higher lower extremity stiffness is associated with
increased risk of Achilles injury is needed. If a link
between stiffness and injury is identified, then the next step
will be to identify the exact causes of increased stiffness
and injury risk, for example, tendon stiffness, muscle
strength or coordination. Variability of stress to better
distribute the load on the muscle–tendon unit is needed.
Clinicians should investigate the amount of high inten-
sity or speed work and running on soft surfaces that an
athlete has incorporated into training, in those presenting
with a new or reoccurring Achilles tendon. Advice to limit
speed work and running on surfaces such as sand may be
helpful for these individuals; however, it may increase the
risk of sustaining other injuries. It should be noted that
Stiffness as a Risk Factor for Achilles Tendon Injury
123
increasing stiffness is associated with improved running
performance; therefore, there may be a trade off between
injury risk and performance which should be addressed on
an individual basis.
Acknowledgments Anna Lorimer reviewed the literature as the basis
for her later PhD biomechanical studies on the effects of lower limb
stiffness on running mechanics and injury. Patria Hume as Anna
Lorimer’s PhD supervisor with experience in sports injury biome-
chanics, reviewed the identified literature and edited the manuscript.
Both authors approved the final manuscript.
Compliance with Ethical Standards
Funding This review was funded by the Auckland University of
Technology. Anna Lorimer was funded by a Vice Chandellor’s PhD
scholarship.
Conflicts of interest Anna Lorimer and Patria Hume declare they
have no conflicts of interest relevant to the content of this review.
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